Floods are part of the natural water cycle and flooding is, for
the most part, steered by natural processes.
The immediate cause of the most recent floods, was abnormally
high rainfall combined with unusually mild temperatures, which melted mountain
snows, to produce a massive torrent of water. Over the past century, average
temperatures in southern Germany have increased by between 1°C – 1.5°C.
Rainfall in the Rhine catchment area has risen steadily this century and winter
precipitation has increased by 40%.
The catastrophic flooding might be an early sign of a change in
the climate caused by global warming. The growing concentration of 'greenhouse
gases' could lead to even milder winters in N.W. Europe and hotter, drier
For a large section of its course in Germany, the Rhine flows
through a narrow gorge which restricts the amount of land either side available
to soak up flood water. This increases the flood problem downstream.
Causes of the floods: human causes
Uncontrollable climatic factors are not the only reasons for
the increasingly frequent occurrence of flooding and a number of human-made
factors have made the problem worse.
Apart from the river bed, there are four important factors
which improve the water storage effect of a river's catchment area and help to
control flooding levels:
Vegetation – trees and plants store large quantities of
water and also intercept precipitation before it reaches the ground.
Soil – stores water very effectively and can store up to
one hundred times the quantity of water as vegetation. It behaves like a sponge.
Ground – Steep land does not retain much water. There is
little surface retention in mountainous areas but vegetation on steep land helps
to retain water. By contrast, more water is stored in flat areas.
Drainage Networks – small streams, rivers and their water
meadows fill up and flood when water levels rise, acting as water storage areas.
When water storage in vegetation, soil, ground and drainage
networks is overloaded the drainage situation changes dramatically.
Deforestation in the Alps has reduced interception and soil
storage of water and increased rates of surface runoff.
Urbanisation of the floodplain, with water flowing off roofs
and roads into drains leading directly to the river has greatly increased river
levels after heavy rain.
The Rhine, for several decades, has been put into a kind of
straitjacket. In the past, excess water would flow out over marshes and
floodplains. These acted like sponges, soaking up the water, but since then some
of the land has been drained, cemented and asphalted for buildings and roads.
The embankments have been strengthened and raised to protect
residential and industrial areas, but raising them has closed off former flood
meadows. Steep concrete flood walls along the upstream river banks channel flood
water quickly from the upper reaches of the river but this has shifted the
flooding problems down stream. (Politicians came under pressure to make
riverside land available for local businesses or housing).
The river Rhine is a major shipping highway. To enable larger
barges to use it and to speed up the journey time it has been strengthened,
deepened and canalised. When a storm takes place the flow of water (or
discharge) does not increase straight away. There is a gap, called a 'Time Lag'
between the high rainfall and the peak discharge. A river with a short time lag
and high discharge increases the danger of flooding. Stretches of the Rhine have
been straightened and banks heightened, cutting some 50 kilometres off the
river's 1,320 kilometre meander to the sea. This has doubled the speed of the
water's flow from Basle, at the Swiss border, to Rotterdam. Now, when there is
heavy snow or rain upstream, water cascades down to flood at the mouth or
half-way along, instead of soaking into marshes near its source.
Building Hydro-electric power stations along much of the upper
Rhine has increased the problem. Since the 1950's, the upper Rhine, along the
French-German border, has been changed with the construction of 10
hydro-electric power stations. The 'Power Project' involved building a 'new'
river parallel to the old Rhine and the construction of these H.E.P. stations
created a deeper, faster Rhine.
Changes in farming practices have made fields less absorbent,
as hedges and forests have been chopped down to create prairies farms. The
drainage of swampy areas, and pumping out the ground water for irrigation
purposes have dried out the land even further. The extensive network of cemented
farm roads act as extra drainage channels.
Rhine channel changes 1000-1995
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Sediment carried downstream is deposited and has built up the channel bed,
in places higher than the level of the land.
Abnormally high rainfall over 3 months: the wettest January on record
high spring temperatures caused early snow melt in the Alps - two months
earlier than normal
Rhine gorge reduces floodplain storage capacity
River floodplain upstream narrowed by engineering. Two thousand islands
removed and the river bed lowered by erosion. Floodplain storage capacity
reduced by 60%
Meander bends straightened to improve navigability and speed flow. River 100
km shorter and 30% faster as a result.
Channelizing to accommodate the size and shape of transportation vessels has
speeded flow downstream
Dams upstream trap sediment and speed up the flow downstream
Deflector groynes used to reduce channel width and deepen the navigable
channel have led to faster flow upstream
Levées built for better flood protection
Urbanisation of the floodplain reduces storage capacity and speeds up runoff
Changes in farming practices have reduced floodplain storage capacity
One-third of Holland is below sea level and the drained polders have shrunk,
lowering them below the river level in some places